Generally, calendering of a board is understood as pressing of a board web in a nip formed by two rolls. The diameters and hardnesses of the rolls that form the nip can be either equal or different. The primary function of the calendering of a board is to equalize the thickness profile of the board and to improve the smoothness of the board by pressing the board web against the face of a smooth roll, in which connection, plastic deformations take place in the face of the board web and the roll face is reproduced in the face of the board. Since the compression pressure produced by means of a calendering nip is limited and the time of dwell of the board web in the nip is very short, just of an order of a few milliseconds, the calendering effect is typically intensified by means of a hot calender nip.
A board web is composed of wood fiber, which is a plastic material consisting of various organic polymers, such as amorphous lignin and hemicellulose and partly of crystalline cellulose. When the temperature and the moisture content become higher, the plasticizing temperatures of the fibers become lower.
In calendering, compression of the board web also always occurs, as a result of which the density of the board is increased, i.e., its bulk is lowered, and the rigidity of the board is reduced. In the case of package board, one of the most important pairs of properties are exactly its bulk and rigidity. Package board must be as thick and rigid as possible in relation to its weight. For this reason, calendering of board must be carried out by calendering methods that provide economies in respect of bulk. As a calender for board web, increased use has been made of soft calenders both with coated consumer package grades and also with other board grades. In soft calenders, attempts are made to take advantage of the higher calendering temperature and an extended nip in order to improve the deformability in the calendering of board. For example, attempts are made to abandon Yankee cylinders completely by replacing them with soft calenders. Also, owing to some of their inferior properties, attempts are made to abandon so-called wet-stack calenders, which have been used for calendering of board, by replacing them with soft calenders.
It is an ordinary arrangement in a production line, which comprises surface sizing and/or coating and calendering of a board web, that the board web is first passed, for example, through a film size press, and after that the board web is dried in a dryer group which comprises a number of drying cylinders. After the drying unit placed after the film size press, calendering of the board web is carried out, usually by means of a so-called wet-stack calender. A wet-stack is used primarily because in the preceding drying unit, the board web has been dried to an excessive dryness, in which case it must be moistened in the calender to the desired water content. In a wet-stack calender, water is added to the face of the board either by means of a mist jet before the first nip or by means of a water doctor arranged in connection with one or more of the calender rolls. By means of the water, exclusively the surface layer of the board web is moistened, in which case it is possible to achieve good smoothness without compressing the board so that it becomes excessively thin. A wet-stack calender commonly consists of a number of rolls that are arranged as a stack of rolls whereby adjacent rolls form a nip with one another.
After the calendering, the board web is dried again, because the amount of water that was added to the board web in the wet-stack calender does not evaporate from it to a sufficient extent during calendering, in which connection, after calendering, the drying is again carried out in a dryer group which comprises a number of drying cylinders. After this second drying, the board web is finally passed through a machine stack. From the above, it may be appreciated that the conventional arrangement requires a large amount of space in the machine direction, because surface sizing has required drying equipment of its own, and calendering in the wet-stack calender has required its own drying equipment. It has been desirable to avoid using such an abundant requirement of space for a calender.
In view of producing an adequate moisture gradient, in a number of board machines, a steam box is installed additionally in front of the calender in order to moisten the surface layer of the board web. In moistening by means of a steam box, the high temperature of the web is a considerable problem, for this temperature ought to be lowered to a level considerably below about 70.degree. C. in order that a sufficiently large amount of steam may condense in the face of the board web. This again requires cooling of the web, which again causes further problems because of the requirement of space and the dripping and problems of rust arising from the cooling cylinders. Cooling of the web by other means, except by means of cylinders, is economically unprofitable and technically difficult.
So far, it has not yet been fully possible to provide a solution in which a wet-stack calender could be replaced completely by means of a soft calender. Attempts have also been made to develop a soft calender in the direction towards gradient calendering. The materials of a soft roll have been developed so as to endure higher temperatures, and the nip temperature has been raised constantly, and the development seems to be moving further towards ever higher temperatures. These very high temperatures produce difficulties of their own in calendering.